This invention relates to boost switching power conversion and to input current control.
The proliferation of AC line-operated electronic equipment in the office, factory and home (e.g. personal computers, workstations, FAX machines, controllers) has caused equipment designers, equipment end-users and utility companies to pay increasing attention to the interface between the equipment and the AC utility lines. Conventionally, most line-operated electronic equipment has incorporated a simple capacitive-input rectifier circuit for processing the AC line voltage into a raw source of DC voltage from which system loads draw their power (in many cases, the system loads are postregulators, such as DC-DC converters, which process the relatively high voltage rectifier output into the lower voltages appropriate for powering equipment circuitry). There are a number of disadvantages associated with using capacitive-input rectifiers. Capacitive-input rectifiers draw relatively high peak currents for a relatively short time period at the peak of each half-cycle of the AC line. The high harmonic content of this current waveform results in an rms current which is much higher than it would otherwise be if the waveform of the current followed the essentially sinusoidal waveform of the utility line. Typical capacitive-input rectifiers exhibit a power factor (e.g. the ratio of the average power delivered to the equipment by the rectifier to the product of the rms values of the AC-line voltage and line current) in the range of 0.5 to 0.65. To the designer of electronic equipment, power factor translates directly into how much packaged electronic circuitry can be powered off of a standard utility outlet. If equipment with unity power factor is plugged into a standard outlet rated to deliver 1200 Watts, then the equipment can utilize the full rating the outlet. If however, the equipment has a power factor of 0.5, only 600 Watts of real power can be utilized, the rest being lost at the utility interface in the form of circulating harmonic currents.
One prior art method of achieving essentially unity power factor at the utility line interface involves interposing a boost switching power converter (i.e. a switching power converter which delivers power to a load at a voltage level greater than the voltage level of its input source) between the output of a line rectifier and the system loads. A unity power factor controller maintains the boost converter output voltage at an essentially constant level, above the maximum anticipated peak value of sinusoidal line voltage, while simultaneously forcing the waveform of the current drawn from the AC line to follow the line voltage waveform. When compared to conventional capacitive-input rectifier schemes of equivalent power, unity power factor preregulators allow for a substantial reduction in the size of storage capacitors, significantly reduce the peak and rms currents drawn from the AC line, and provide for essentially full utilization of the power rating of standard AC power outlets. Examples of AC to DC converters of this type are described in Wilkerson, U.S. Pat. No. 4,677,366 and Williams, U.S. Pat. No. 4,940,929.
Also, in prior art unity power factor preregulators the magnitude of the converter input current is sensed by an element having a fixed transfer characteristic (e.g. a resistor or a current transformer), and this current sense signal is used by control circuitry to force the input current to follow the line voltage waveform. As converter load is decreased, both the converter input current and the magnitude of the signal delivered by the current sensing element decrease. As a result, as load is decreased, second order effects (e.g. amplifier offset voltages, system switching noise) become increasingly significant relative to the magnitude of the current sense signal and power factor control performance is degraded.